Linking PFC surface characteristics and plasma performance in the Lithium Tokamak Experiment

The Lithium Tokamak Experiment (LTX) is a spherical torus magnetic confinement device designed to accommodate lithium as the primary plasma-facing component (PFC). Results are presented from the implementation on LTX of the Materials Analysis and Particle Probe (MAPP), a compact \textit{in vacuo} surface science diagnostic. With MAPP, \textit{in situ} surface analysis techniques of x-ray photoelectron spectroscopy and thermal desorption spectroscopy are used to study evolution of the PFC surface chemistry in LTX as a function of varied lithium coating, hydrogen plasma exposure, and PFC surface temperature ($20-300\,^{\circ}C$). Surface analysis results are then correlated with various measures of LTX plasma performance, including toroidal plasma current, line-integrated plasma density, and density-normalized impurity emission. Lithium coatings are observed to convert within hours to $Li_2O$ by gettering oxygen from both the residual vacuum and the PFC substrate. However, plasma performance remains elevated even with discharges operating against $Li_2O$-coated PFCs. Hydrogen is retained by these $Li_2O$ coatings during a discharge, but it is almost completely desorbed as outgassed $H_2$ in the minutes following the discharge; no persistent $LiH$ formation is observed.